Scheme of power enhancement for  combined cycle plants through steam injection

ABSTRACT

A combined cycle power plant has his power output augmented by addition of a gas turbine (GT 1 ) with a dedicated heat recovery steam generator (HRSG 1 ). Steam produced by said heat recovery generator (HRSG 1 ) is injected in the combustion chamber of the combined cycle plant. In this way the combined cycle plant power is augmented by the power of the additional gas turbine plus two further contributions due to steam injection. The first contribution is given by the increased flow rate through the gas turbine (GT) of the combined cycle plant. The—second contribution is given by the said increased flow rate flowing also through the heat recovery steam generator (HRSG) of the combined cycle plant, hence increasing the thermal power available herein and causing an increased power output at the steam turbine (ST). With said method a significant power increase can be obtained by a given combined cycle plant, with good energy conversion efficiency and excellent load following capability, thus allowing a complete and effective exploitation of sites devoted to power production.

TECHNICAL FIELD

The present invention concerns the field of energy conversion and is aimed at the performance enhancement of power plants. More particularly, this invention relates to a combined cycle power station whose performance are enhanced through injection of steam produced from waste-heat of a gas turbine.

BACKGROUND ART

Fossil fuelled power plants are nowadays basically divided into 4 categories: reciprocating internal combustion engines, gas turbine plants, steam turbine plants and combined cycle plants.

Internal combustion engines for electric power have high efficiency but small unit power. Gas turbine plants have higher unit power, but are impaired by lower efficiency, which is made evident by the relevant energy loss at turbine exhaust.

Steam turbine plants have very high unit power, but are very complex and expensive, both during construction and in use.

Combined cycle plants are at top of energy conversion efficiency and give a unit power close to steam plants. Moreover, they are somewhat simpler than steam plants. Therefore, accounting for capital and operation cost, they give at now the lowest cost of electricity produced from fossil fuels. According to IEA, in OECD countries, combined cycle plants have seen in recent years an annual growth rate of slightly less than 20 percent, which is about four times that of gas turbines and 10 times that of steam turbines.

As shown in FIG. 1, combined cycle plants are composed of a gas turbine plant GT that powers a load U1 and meanwhile delivers the residual energy of its exhaust flow to a heat recovery steam generator (HRSG). This residual energy is hence partially recovered in a steam turbine ST, which can be either fitted on the same axis of the GT or connected to his own load U2. In some plants the steam generator HRSG hosts a supplemental combustion in order to raise the energy available to the ST. Steam exhausted by the steam turbine is condensed and brought back to the HRSG. This HRSG can be fitted with several steam paths at different pressure levels, in order to optimize the thermal energy recovery from GT exhaust. The steam turbine can be divided into sections and the exhaust from the high pressure section can be brought back to the HRSG, in order to raise available energy for subsequent sections. FIG. 1 shows a schematic state-of-the-art configuration of a combined cycle power station.

An alternative to combined cycle plants is given by steam injected gas turbines (STIG). In these plants (FIG. 2), part of the available energy at a GT exhaust is recovered in a HRSG. Steam produced in this generator is injected in the combustion chamber or at the compressor discharge of the GT. Thereby the flow rate through the turbine is raised and hence its power is augmented. Efficiency is increased as well, though not as much as in combined cycle plants.

Another case is that of “repowered” steam plants, which have been used in the past to face the lack of available locations for new power plants. In this case a gas turbine was added to an existing steam plant delivering power and high temperature exhaust. This exhaust gas was used in various ways inside the steam plants, directly as oxidant in the steam generator (“boiler repowering”) or as heating fluid in the regenerative line (“feed-water repowering”). However steam plant repowering has been impaired by heavy efficiency limitations, as well as by constructive difficulties and limited power increase.

BRIEF DESCRIPTION OF DRAWINGS

The invention will be described in more detail in the following text with reference to the figures, in which:

FIG. 1 shows a schematic state-of-the-art configuration of a combined-cycle power station;

FIG. 2 shows a schematic state-of-the-art configuration of a gas turbine with steam injection (STIG);

FIG. 3 shows a possible configuration of the invention.

DISCLOSURE OF INVENTION

The invention is aimed at increasing the power of combined cycle power plants. FIG. 3 shows on the right a combined cycle plant similar to that of FIG. 1, i.e. composed of:

-   -   a) a gas turbine plant GT of whatever kind and configuration,     -   b) a heat recovery steam generator HRSG having one or more         pressure levels, either with or without steam re-heat and         firing,     -   c) a steam turbine plant ST fed by the steam produced in the         HRSG.

Power enhancement of this base plant is obtained via addition of a further gas turbine GT1. The energy contained in the exhaust flow me1 of said GT1 is recovered by a further heat recovery steam generator HRSG1. The steam produced by such HRSG1 is introduced in the combustion chamber or at the compressor discharge of the gas turbine GT in the base plant, thereby integrating the base and the additional part of the plant.

As a consequence of said steam injection, the gas turbine GT of the base combined cycle plant increases its mass flow rate, hence delivering an increased power to its load U. This increased flow rate feeds the heat recovery steam generator HRSG of the base plant, which in turn increases the thermal energy made available to the bottoming steam cycle ST. Therefore the latter increases its power output as well.

Main achievements of the proposed configuration are:

-   -   1. The integration of the additional section doesn't require         substantial modification of the base plant.     -   2. The base plant continues its operation without any efficiency         penalty.     -   3. The additional gas turbine GT1 gives a significant power         gain.     -   4. A further power gain is available at the gas turbine GT of         the base plant due to the increased mass flow rate caused by         steam injection in the combustion chamber or at the compressor         discharge.     -   5. A still further power gain is available at the steam turbine         ST of the base plant due to the increased mass flow traveling         from the gas turbine through the steam generator HRSG.     -   6. The global power of the plant modified as above described can         be increased by over 50% from its base value.     -   7. The global energy efficiency of the modified plant is close         to its value of the base combined cycle plant.     -   8. The additional section is composed of a gas turbine GT1 and a         heat recovery steam generator HRSG1. The latter has a very         simple structure, being its operating pressure not too far from         that of the combustion chamber of the base gas turbine GT. This         pressure is therefore well below the usual values employed in         HRSGs, allowing the uses of a single pressure level. Everything         considered, the additional section has a modest capital cost and         doesn't increase significantly the construction, management and         maintenance cost of the plant.     -   9. Even if the additional section is by far less complex of a         combined cycle plant, the additional power is gained with a         limited fuel consumption, well below of the values which would         be featured by the additional gas turbine in stand-alone         configuration. This advantage is due to the optimum integration         obtained by the proposed scheme.     -   10. Existing combined cycle plants have an average residual life         which is well longer than any other existing power plant.         Therefore, when the intervention is carried on an existing         plant, its cost can be recovered on a longer period.     -   11. The whole additional section can be partialized for load         modulation or even excluded. Therefore the modified plant has an         outstanding operating flexibility. If the additional gas turbine         GT1 is fitted with variable intake guide vanes (IGV), its power         can be reduced with limited efficiency penalty through a wide         range.     -   12. The partialization of the additional section can be         integrated with the part load capability of the base plant. If         the gas turbine of the latter is fitted with variable intake         guide vanes, optimal strategies can be envisaged in order to         have a even wider part load range with small efficiency penalty.     -   13. The power increase gained by the proposed scheme is obtained         with an efficiency which can be comparable to that of a complete         combined cycle, even if the added components are by far simpler         and less costly than such complete plant.     -   14. Accounting for both capital and fuel cost, the cost of         electricity produced by the modified plant is roughly equal to         that of the base combined plant.     -   15. The additional section can be set up without a prolonged         shut-off of the baseline plant, a part from the modification of         the combustion chamber of the latter. This reduces the expense         of modification in case of existing plants.     -   16. The steam absorbing capacity of a large scale combined cycle         is such that the whole exhaust energy from a heavy-duty gas         turbine can be recovered. Use of a heavy-duty gas turbine         guarantees the minimum operating cost.     -   17. Smaller gas turbines, like aeroderivative units, can be         employed as well. In this case the additional units can be more         than one, enhancing thereby the operating flexibility of the         modified plant as the additional gas turbines can be partialized         or excluded sequentially.     -   18. If the additional gas turbine GT1 has been designed         according to most recent practice for polluting emission         reduction, the specific pollutant production of the modified         plant is reduced or at least unchanged with respect to the base         combined cycle plant. Furthermore, the base combined cycle plant         has a benefit in terms of nitrogen oxide emissions, due to steam         injection in the combustion chamber.

If the proposed scheme were extensively applied on existing combined cycle power plants, either for pure power or for combined heat and power production, a significant part of the power demand increase could be covered without exploiting new locations, hence avoiding the related environmental impact. Global average energy efficiency would gain a benefit comparable to that achieved by new combined cycle plants. Load following capability, on both daily and seasonal time-scales, would also be enhanced.

LIST OF SYMBOLS

-   GT Gas turbine -   ST Steam turbine -   HRSG Heat recovery steam generator -   U Load -   me Exhaust flow -   ms Steam flow -   1 Referred to the additional section 

1. A method for increasing the power of a combined cycle power plant, i.e. a power plant containing at least a gas turbine plant GT, at least a heat recovery steam generator HRSG and at least a steam turbine plant ST, which method comprises: a. the addition to said combined cycle plant of a gas turbine GT1 delivering a first power contribution to the load U1; b. the addition of a heat recovery steam generator HRSG1 fed by the exhaust energy delivered by the gas turbine GT1; c. the injection of the steam produced by the generator HRSG1 in the combustion chamber or at the compressor discharge of the gas turbine GT of the base plant. This steam injection, within the power limits posed by design, increases the power delivered by the gas turbine GT to its load U; d. an increased flow rate through the heat recovery steam generator HRSG of the base plant, due to said steam injection, and consequently an increased power delivered by the steam turbine ST of the base plant to its load U.
 2. A method for integration of a gas turbine and a combined cycle plant, either existing or in commissioning or in design phase, in which method the exhaust energy of said gas turbine is used to produce steam for subsequent injection within the gas turbine group of said combined cycle plant, as described in claim 1, and said integration allows the achievement of a global power exceeding the sum of those delivered by the two plants that are integrated, while the energy efficiency of the integrated plant is close to that of the combined cycle and significantly higher than that of the gas turbine in stand alone configuration.
 3. The method as described in claims 1 and 2 in which the additional gas turbine TG1 is a single high-power unit, so that the high flow rate and the high temperature of the exhaust flow allow a large steam production and reach the upper power limit of the combined cycle plant given by its design constraints in terms of steam injection allowance.
 4. The method as described in claims 1 to 3 in which, instead of a single gas turbine TG1, more than one additional gas turbines with limited power are employed, in order to achieve higher operating flexibility.
 5. The method as described in claims 1 to 4 in which any kind of gas turbine, either heavy duty or aeroderivative, independently from their technology or unit power, is employed as additional unit.
 6. The method as described in claims 1 to 5 in which the additional heat recovery steam generator HRSG1 may have a simplified configuration with respect to other generators commonly employed in combined cycle power plants, according to what follows: a. the operating pressure is set at the minimum value needed in order to inject the steam in the combustion chamber or at the compressor discharge of the gas turbine in the base plant; as such, said operating pressure can be significantly lower than that normally used in the steam generators which feed a steam turbine; b. given the said limited operating pressure, the recovery of the thermal energy available at the gas turbine GT1 exhaust can be effectively achieved with a single pressure level, keeping minimum temperature difference at pinch point.
 7. The method as described in claims 1 to 6 applied to combined cycles however conceived, either producing electricity or combined heat and electricity from any kind of fuel, whichever is their configuration, i.e. whatever is the number of bottoming steam cycles or the number of pressure levels in the heat recovery steam generator, either with or without supplemental firing in said heat recovery steam generator.
 8. The method as described in claims 1 to 7 in which, as an alternative, a part of the steam produced by the heat recovery steam generator HRSG1 is injected in the combustion chamber or at the compressor discharge of the additional gas turbine GT1, so that also this additional gas turbine has an increased power output either in design or in off-design conditions.
 9. The method as described in claims 1 to 8 in which the additional heat recovery steam generator HRSG1 may be fitted with a supplemental firing, in order to increase the steam flow to be injected in the gas turbine GT of the base plant or in the additional gas turbine GT1.
 10. The method as described in claims 1 to 9 in which the power output can be regulated as follows: a. the additional gas turbine GT1 has variable intake guide vanes in its foremost compressor stages, so that the power output of said GT1 can be reduced as well as its exhaust flow rate; b. the exhaust energy transferred to the additional heat recovery steam generator HRSG1 is therefore reduced; c. the steam injected in the gas turbine GT of the base plant is reduced as well, bringing back the global power of the modified plant to any value between the base level and the maximum level of the modified plant.
 11. The method as described in claims 1 to 10 in which the whole additional section GT1/HRSG1 can be switched off in case of low load request, bringing back the combined cycle plant to its original values of power and efficiency.
 12. The method as described in claims 1 to 11 in which the additional section has more than one unit GT1/HRSG1 and these units can be excluded in sequence, allowing a number of intermediate power levels.
 13. The method as described in claims 1 to 12 in which both the additional gas turbine GT1 and the gas turbine GT of the base plant are fitted with variable intake guide vanes and an optimal sequence of intervention is studied for the two sets of variable intake guide vanes in order to allow the widest power range with the minimum penalty in terms of efficiency.
 14. The method as described in claims 1 to 13 in which power regulation by fuel flow rate reduction is used only after complete exploitation of the other techniques described in claims 10 to 13, in order to give a further widening of the operating range at the expense of some efficiency penalty.
 15. The method as described in claims 1 to 14 in which any power level between that of the base plant and that of the modified plant can be attained without excessive penalty in terms of energy efficiency by optimal use of: a. variable intake guide vanes of one or more compressors; b. switching off one or more additional gas turbines; c. reducing fuel flow rate in one or more combustion chambers; as well as any other power regulation system. 